1. The Field of the Invention
The principles of the present invention relate to a class D amplifier implemented as an H-bridge that implements switching using one or more intermediate switching phases.
2. The Relevant Technology
Electronic circuitry provides complex functionality that is proving ever more, useful. Electronic circuitry pervades our modern lives in areas such as communication, entertainment, travel, productivity, and the like. One common type of circuit is the class D amplifier, which performs switching operations to generate a pulse signal. If an input signal increases to have a higher voltage than a reference voltage, the output pulse signal transitions from one binary state to another binary state. If the input signal decreases to have a lower voltage than a reference voltage, the output pulse signal transitions back from the other binary state to the original binary state.
Class D amplifiers are known to be highly efficient, and can have the form of an H-bridge. FIG. 10 illustrates a conventional H-bridge 1000. The conventional H-bridge includes four switches 1001 through 1004, an inductor 1005 and a resistor 1006. Switches 1001 and 1002 may be, for example, p-type Field Effect Transistors (FETs). Switches 1003 and 1004 may be, for example, n-type FETs.
In order to invoke one binary output state, switches 1001 and 1004 are closed, and switches 1002 and 1003 are open, allowing current to flow from high voltage supply Vdd through switch 1001, inductor 1005, resistor 1006 and switch 1004, into the low voltage supply −Vdd (i.e., minus Vdd). Thus, a high voltage is generated on node 1011 which is at or close to Vdd, whereas a low voltage is generated on node 1012 which is at or close to −Vdd (minus Vdd). This relies on the impedance of the series combination of the inductor 1005 and the resistor 1006 being sufficient such that it dominates over the impedance through the switches. In order to invoke the opposite binary state, switches 1001 and 1004 are open, and switches 1002 and 1003 are closed, allowing current to flow from high voltage supply Vdd through switch 1002, resistor 1006, inductor 1005, and switch 1003 into low voltage supply −Vdd (i.e., minus Vdd). Accordingly a high voltage is generated on node 1012 which is at or close to Vdd, whereas a low voltage is generated on node 1011 which is at or close to −Vdd (minus Vdd).
Thus, the voltage at the output nodes 1011 and 1012 swings rapidly between Vdd and −Vdd (minus Vdd) relatively quickly during a switching operation for a total voltage change of nearly twice Vdd. FIG. 11A shows the transition of the voltage at intermediate node 1011 during a high-to-low switch operation, whereas FIG. 11B shows the transition of the voltage at intermediate node 1011 during a low-to-high switch operation. Note the rapid change of voltage between Vdd and −Vdd (minus Vdd)
There are often power losses during a switching operation (often referred to as “switching losses” or “fCV2” losses). As implied by the term “fCV2” losses, the power loss is proportional to the operational frequency of the H-bridge (f), the drain parasitic capacitances on the output nodes 1011 and 1012 (C), and the square of the voltage change (V2).
Thus, even though class D amplifiers such as the H-bridge are highly efficient, it would be advantageous to improve power efficiency even further to thereby reduce heat generation and operational costs.